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Intimidation, formerly Instill Doubt, is a special attack which fills your target with fear. If successful, the mob will flee just as with the spell Fear. The skill has a low chance to succeed compared to most skills, and even if it succeeds, partial resists (in which the mob flees for a shorter than maximal duration) or full resists are possible. These are likely connected to magic resist since a full resist calls it a "Fear spell". The skill has a cooldown of 10 seconds but is affected by haste.
It can be used regardless of line of sight and distance as long as you have an NPC targeted. There is no chance of a successful intimidate when performed this way but it will skill up nonetheless, making training this skill safe and trivial almost anywhere.